The present invention relates to a high voltage noise filter for use in microwave devices and a magnetron device incorporating such a high voltage noise filter.
FIG. 10 is a bottom end view illustrating a relevant portion of a magnetron incorporating a conventional high voltage noise filter 1. In the figure, stem terminals 5a and 5b are connected with conductive wires 2a and 2b partially wound into coils, respectively, for prevention of leakage of fundamental wave noise and higher harmonic noise from the respective stem terminals 5a and 5b of the magnetron. The conductive wires 2a and 2b are coated with an insulating material along the overall length thereof except for end portions 30a and 30b connected with the respective stem terminals 5a and 5b. At coil portions defining choke coils 4a and 4b, and the conductive wires 2a and 2b are coated with insulating layers 1a and 1b. At the other coil portions 3a and 3b of the respective conductive wires 2a and 2b, conductive layers 34a and 34b, shown by diagonal hatching, are formed on the aforesaid insulating layers 1a and 1b. The conductive wires 2a and 2b have only the insulating layers 1a and 1b at end portions 31a and 31b of the coil portions 3a and 3b, and terminals thereof are connected to respective fastening tabs (terminals) 7a and 7b to be connected to a microwave source 6. The coil portions 3a and 3b are accommodated in cylindrical conductive members 8a and 8b, respectively. The conductive layers 34a and 34b electrically contact the interior walls of the respective cylindrical conductive members 8a and 8b and the coil portions 3a and 3b are mechanically secured in the respective cylindrical conductive members 8a and 8b. FIG. 10 shows a cross-section of the cylindrical conductive members 8a and 8b for illustration of the coil portions 3a and 3b. The cylindrical conductive members 8a and 8b are accommodated in resin casings 16a and 16b. Within the resin casings 16a and 16b, the conductive members 8a and 8b accommodating the coil portions 3a and 3b, the terminal portions 31a and 31b and joints between the fastening tabs 7a and 7b and the terminal portions 31a and 31b are filled with an insulating resin material 11. The conductive members 8a and 8b are electrically connected with a filter casing 10 by way of grounding plates 9.
In the high voltage noise filter of such a construction, the insulating layers 1a and 1b formed on the outer peripheral surfaces of the conductive wires 2a and 2b exert a significant influence on the withstand voltage characteristics of the high voltage noise filter. Particularly, terminal regions 1e and 1f of the coil portions 3a and 3b may sometimes suffer an abnormal increase in the electric field strength in the vicinity of the end portions of the conductive layers 34a and 34b, which increase in the electric field strength tends to cause insulation breakdown near the end portions of the conductive layers. The prevention of the insulation breakdown requires an increase in the withstand voltage characteristics of the insulating layers 1a and 1b of the conductive materials 2a and 2b. Unfortunately, an increase in the withstand voltage characteristics requires the insulating layers 1a and 1b to have an extreme thickness. An increased thickness of the insulating layers 1a and 1b leads to an increased size of the coil portions 3a and 3b. This results in not only a larger size of the high voltage noise filter but also decrease in the capacitance between the outer peripheral surfaces of the coil portions 3a and 3b and the conductive wires 2a and 2b. Consequently, the noise filter suffers a serious deterioration of the characteristics thereof.